Rebuild the Journal home around a public feed and flagship marketing

The old home was an h1, subtitle, and two auth buttons — visitors
arriving at trails.cool had no idea what it was, and self-hosted
instances had nothing interesting to land on.

The new home is one layout that serves both audiences:

- Hero: a product-describing h1 ("Federated outdoor journal") +
  tagline. The site name already lives in the top banner and nav brand,
  so the h1 carries the pitch instead of triplicating "trails.cool".
- Auth CTAs: Register (blue) + Sign In (outlined) get primary weight.
  Planner demotes to a small "Or try the Planner without an account →"
  link below them — it's a nice escape hatch but not the goal.
- Marketing cards: on the flagship only, a 2x2 grid of emoji-icon cards
  for Planner / Journal / Federation / Ownership, matching the Planner
  home's visual weight. `IS_FLAGSHIP=true` toggles it. Self-hosters get
  a "Powered by trails.cool — about the project" link back to the
  flagship instead, so they don't have to write their own marketing.
- Public feed: reverse-chronological list of the 20 most recent public
  activities on the instance, with owner + distance + date. Empty state
  for fresh installs.

Plumbing:
- `listRecentPublicActivities(limit)` in activities.server.ts joins
  users so the feed can render "by <displayName>" in one query.
- `IS_FLAGSHIP` env wired through docker-compose.yml; cd-apps and
  cd-infra both write `IS_FLAGSHIP=true` alongside `DOMAIN=trails.cool`,
  so self-hosters (who deploy without these workflows) default to off.

Specs:
- `activity-feed`: new "Instance-wide public activity feed" requirement
  so the visibility contract is pinned (public in feed, unlisted +
  private are not).
- `journal-landing`: new small spec capturing the hero / marketing /
  feed layout contract and the `IS_FLAGSHIP` toggle, so future
  instance-branding or empty-state changes have a stable anchor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infrastructure/docker-compose.yml

@@ -23,6 +23,11 @@ services:
       DOMAIN: ${DOMAIN:-trails.cool}
       ORIGIN: https://${DOMAIN:-trails.cool}
       PLANNER_URL: https://planner.${DOMAIN:-trails.cool}
+      # "true" only on the flagship trails.cool instance. The Journal
+      # home renders the project marketing block when set, and the
+      # "Powered by trails.cool" footer link when not. Default empty =
+      # self-host, which is the safer default.
+      IS_FLAGSHIP: ${IS_FLAGSHIP:-}
       DATABASE_URL: postgres://trails:${POSTGRES_PASSWORD:-trails}@postgres:5432/trails
       JWT_SECRET: ${JWT_SECRET:-change-me-in-production}
       SESSION_SECRET: ${SESSION_SECRET:-change-me-in-production}